Influence of Sedimentary Environments on Organic Matter Enrichment in Marine-Continental Transitional Shale: A Case Study of the Upper Permian Longtan Formation in the Central Hunan Depression, China

Abstract

The shales of the upper Permian longtan formation (LTF) in the central Hunan area have a wide distribution and high organic matter content. Unlike the anoxic conditions of marine shales, the LTF marine-continental transitional shales were deposited in an oxic−dysoxic environment. Under such a depositional background, LTF shale is still enriched in organic matter, and the controlling factors of the sedimentary environment on the organic matter enrichment shale remain unclear. A series of geochemical analyses of 49 shale samples from well XSD1 were used to characterize the paleoenvironments of shales in different sections of the LTF and identify mechanisms of organic matter enrichment. Various geochemical indicators show that in the organic matter-rich sections, the warm and humid paleoclimate accelerated the weathering rate of the source area, and the high terrestrial debris flux brought a large amount of nutrients into the water body, promoting the proliferation of planktonic bacteria and algae and leading to higher paleoproductivity in the surface water. Although the oxic−dysoxic bottom water was not favorable for the preservation of organic matter, the high settling rates reduce oxidative decomposition of organic matter in the water column, leading to rapid burial and enrichment of organic matter. Particularly, redox proxies (e.g., Mo EF and U EF ) indicate that foraminiferal-containing shales were deposited with high levels of dissolved oxygen in the water column, which is unfavorable for the enrichment of organic matter. The formation process of siderite consumed large amounts of organic matter, which was not conducive to the enrichment of organic matter.